1. Field of the Invention
The present invention relates to liquid crystal display device field, in particular, to a method for driving an LED backlight source in a liquid crystal display device.
2. Description of the Prior Art
A liquid crystal display (LCD) presenting advantages over thinness, low power consumption and non-radiation, is widely used. Most LCDs in present market are backlight type LCDs, comprised of liquid crystal panel and backlight module. The liquid crystal panel has liquid crystal molecules between two paralleling glass substrates. Light from the backlight module is refracted depending on an alignment of the liquid crystal molecules controlled by voltage drop between the two glass substrates through many vertical and horizontal tiny wires thereon, thereby displaying images. The backlight module is one of the key components of LCD device, because normal display of image is provided by light from the backlight module, for the liquid crystal panel does not illuminate itself. According to light incident position, the backlight modules are classified as side-in type and direct-in type. The direct-in type backlight module emits light to the liquid crystal panel by forming surface light source through setting up the light sources such as Cold Cathode Fluorescent Lamps (CCFLs) or Light Emitting Diodes (LEDs) in the rear of the liquid crystal panel. The side-in type backlight module emits light to the liquid crystal panel by forming surface light source through setting up backlight LED lightbars on the bezel fringe in the side rear of liquid crystal panel. The surface light source is generated in the process in which the light from the LED lightbars is incident to a light-in surface of a Light Guide Plate (LGP) and departs from a light-out surface of the LGP after reflection and refraction, and optical films.
Please refer to FIG. 1 illustrating a circuit diagram of an LED backlight driving circuit for use in an LCD having 2D and 3D modes. A constant current supplying chip IC (constant current supplying chip) 300 has an OVP (output overvoltage protection) pin, in which a voltage comparator 200 having resistors R100 and 8200 in series divides voltage for driving LED series circuit. When voltage applied on the resistor 8200 is higher than internal constant voltage source (generally 2V) in constant current supplying chip IC 300, the constant current supplying chip IC 300 turns off a field effect transistor (FET) Q100, as a consequence output voltage (i.e. driving voltage for LED series circuit 100) stops rising, so as to protect components of the backlight driving circuit from burning out. Current flowing over LED series circuit 100 varies linearly with voltage needed. As to LCD having 2D and 3D mode, the peak value of backlight LED driving current under 3D mode is higher, and voltage needed is correspondingly higher. Calculating on basis of 8 LEDs in a LED series circuit, voltage needed under 3D mode is 10V higher than that under 2D mode, henceforth, the overvoltage protection point is determined as 1.2 times of driving voltage needed of LED series circuit under 3D mode. If the overvoltage protection point is determined by driving voltage needed of the LED series circuit 100 under 2D mode, the LED series circuit under 3D mode may be short of driving voltage to illuminate normally.
However, determining the overvoltage protection point by driving voltage of LED series circuit 100 under 3D mode is technically defective as described below: when overvoltage protection is triggered by some unusual causes, if output voltage of driving circuit rises too high, the overall power will augment abruptly, largely affecting the components of the driving circuit (such as fuse melting), and reducing a lifetime of a convertor in the circuit (such as transformers).